A fluororesin film and a film made of a fluororesin composite are light in weight and excellent in weather resistance, mechanical strength, etc. and therefore used as a covering material for e.g. a swimming pool, a gymnasium, a tennis court, a football ground, a warehouse, a hall, an exhibition hall, a horticultural greenhouse or an agricultural greenhouse (for example, JP-A-3-188132, JP-A-63-222852). However, a film made of a composite of glass fibers and a fluororesin as disclosed in JP-A-63-222852 is insufficient in transparency. Accordingly, if the film is used as a covering material for the roof of a structure, the sunbeam transmittance of the roof tends to be low. Thus, plants in the interior of the structure will not sufficiently grow. Accordingly, the above film made of the composite can not be applicable as a covering material for horticultural facilities or sports facilities in which grass grows. Further, the interior of an architectural structure will be insufficient in wide open feeling. Here, an architectural structure wherein a membrane material is used for the roof or the outer wall will be referred to as a membrane structure. Further, a film which is fixed to a framework of e.g. iron or wood to form the roof or the outer wall portion of a structure will be referred to as a film for a membrane structure.
An ethylene/tetrafluoroethylene copolymer (hereinafter referred to as ETFE) film is excellent in the sunbeam transmittance and is thereby used as a film for a membrane structure suitable for e.g. an agricultural greenhouse, a horticultural greenhouse (see JP-B-08-5976), sport facilities in which grass grows and an exhibition hall. International Conference on Building Envelope Systems and Technology 2nd., Setphen Tanno et al., 1997 proposes a roof obtained in such a manner that a plurality of films for a membrane structure overlaid one on another are sandwiched between an iron frame to form a panel, and air is put between the films to blow the panel up. Such a film for a membrane structure is required to have mechanical strength such as tensile strength and tear strength as well as weather resistance and transparency. Further, the film for a membrane structure is exposed to direct sunlight in summer (Architectural Institute of Japan, AIJ Recommendations for Loads on Buildings) and thus it is required to maintain excellent mechanical strength even at a high temperature of about 60° C.
As an ETFE excellent in mechanical strength, a tetrapolymer ETFE obtained by copolymerization of from 40 to 60 mol % of ethylene (hereinafter referred to as E), from 30 to 55 mol % of tetrafluoroethylene (hereinafter referred to as TFE), from 1.5 to 10 mol % of hexafluoropropylene (hereinafter referred to as HFP) and from 0.05 to 2.5 mol % of a comonomer as a fourth component has been proposed. It is excellent in mechanical strength as compared with a terpolymer ETFE having no fourth component comonomer copolymerized (JP-A-57-38807). However, for a film for a membrane structure, more excellent mechanical strength and transparency are required. A tetrapolymer ETFE excellent in transparency and tear strength has been proposed (WO98/10000), but its mechanical strength has been significantly dependent on temperature, and its mechanical strength at high temperature is not necessarily sufficient.